1. Field of the Invention
This invention pertains to the field of calcination of inorganic minerals and materials. In particular, it relates to a self-contained, thermally insulated flash calcination unit that promotes reaction temperature control and reduces energy consumption.
2. Description of the Related Art
Carbonate and hydrate inorganic minerals, such as limestone, dolomite, magnesite, brucite, and other materials, are subjected to calcination to produce oxides for industrial utilization. The process of calcination has been carried out in a variety of vessels, ranging from rotary kilns to fluidized-bed reactors, with a high input of energy dedicated to maintaining the relatively high temperature required for calcination.
For instance, limestone is converted to calcium oxide and carbon dioxide in an endothermic decomposition reaction that is carried out optimally at a temperature of at least 1,700.degree. F. The required heat is typically provided by gas burners in the reaction zone. The prior art discloses several types of equipment for carrying out the calcination of limestone and similar minerals, all directed at improving various aspects of the process. Such inventions are described, for example, in U.S. Pat. Nos., 3,862,294, 4,118,177, 4,747,773, 4,483,831, 4,932,862, 5,132,102, 5,174,749 and 5,260,041.
For convenience, limestone is used in the description of the present invention because of its importance and because it is typical of the feed material subjected to calcination in prior-art processes. In flash calcination processes, limestone particles in a preheated fluid suspension are exposed to a high-temperature flame that activates and sustains the decomposition reaction. If the temperature in the reactor reaches levels above about 2,450.degree. F., sintering of the particles may occur with a resulting degraded product. If the temperature declines below the optimal range of about 1,700 to 2,450.degree. F. prior to completion of the reaction, the quality of the product is again degraded by the presence of unreacted material. Thus, optimal reaction conditions are present when the reactive bed is maintained within the desired temperature range throughout the reactor bed.
This result has been difficult to achieve in all prior-art reactors because of the complex thermal effects produced by the combined dynamics of the fluidized bed, reaction thermodynamics, and heat transfer occurring within the reactor and with the surrounding environment. When the flame temperature is maintained at an optimal level to initiate the reaction, the temperature downstream tends to decrease too rapidly to support full conversion. If, on the other hand, a higher flame temperature is used to provide sufficient heat to maintain an adequate temperature throughout the reactor, some initial sintering of the feed material is found to occur, which is undesirable for product quality. Therefore, there is still a need for improved apparatus that produces more uniform temperature conditions through the reactor than are currently attainable with known equipment.